Tag: applications of ultrasound

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Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

An auditorium of dimensions $(100\times40\times10)\space m^3$ contains $1000\space m^2$ curtains of absorption coefficient $0.2\space m^{-2},\space 2000 m^2$ of carpets of absorption coefficient $0.7\space m^{-2}$. If $1000$ men of absorption coefficients $0.9$ per person are sitting in the hall, then reverberation time is

  1. $2.7\space s$
  2. $7.2\space s$
  3. $3.5\space s$
  4. $3.7\space s$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Volume of the room          $V = 100\times 40 \times 10 = 4\times 10^4      m^3$

Effective surface area of the room           $A = (0.2) 1000  +  (0.7) 2000  +  (0.9) 1000  =  2500$
Now reverberation time       $T _r = 0.161  \dfrac{V}{A}$
$T _r = 0.161  \dfrac{4\times 10^4}{2500} = 2.576    s$

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

A stone is dropped from the top of a tower of $125m$ high into a pond which is at base of the tower. When will the splash be heard at the top? (Given $g = 10ms^{-1}$ and speed of sound $=340ms^{-1}.$)

  1. $5s$
  2. $0.36s$
  3. $5.36s$
  4. $2s$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Time to fall: h = 0.5 * g * t^2 -> 125 = 5 * t^2 -> t^2 = 25 -> t = 5s. Time for sound to travel up: t2 = h / v = 125 / 340 = 0.367s. Total time = 5 + 0.367 = 5.367s.

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

When all of the audience come out of the audotirium the reverberation time became$\frac{4}{3}$ of that when packed with the audience. If all $400$ persons come out, the ratio of the absorption of the auditorium to that a person is 

  1. $1600$
  2. $1200$
  3. $3000$
  4. $1000$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

T = k * V / A. T_empty = k * V / A_auditorium. T_full = k * V / (A_auditorium + 400 * A_person). Given T_empty = 4/3 * T_full. So (A_auditorium + 400 * A_person) / A_auditorium = 4/3. 1 + 400 * A_p / A_a = 1.333. 400 * A_p / A_a = 0.333. A_a / A_p = 400 / 0.333 = 1200.

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

The reverberation time of a hall of volume $200m^{-3}$ is $1.7s$. The reverberation time if $20$ persons having absorptions $0.4$ entered the hall, nearly is 

  1. $1.5s$
  2. $1.4s$
  3. $1.3s$`
  4. $1.2s$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

T1 = 0.161 * V / A1 = 1.7. A1 = 0.161 * 200 / 1.7 = 18.94. New absorption A2 = A1 + 20 * 0.4 = 18.94 + 8 = 26.94. T2 = 0.161 * 200 / 26.94 = 1.195s, which is approx 1.2s.

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

A stone is dropped into a lake from a tower 500 metre high. The sound of the splash will be heard by the man approximately after

  1. 11.5 seconds

  2. 21 seconds

  3. 10 seconds

  4. 14 seconds
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Fall time: 500 = 5 * t^2 -> t^2 = 100 -> t = 10s. Sound time: 500 / 340 = 1.47s. Total time = 11.47s, approx 11.5s.

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

Fill in the blank.

___________ is created when a sound or signal is reflected causing a large number of reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space.

  1. Reverbaration

  2. Echo

  3. Response

  4. Heat
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A reverberation, or reverb, is created when a sound or signal is reflected causing a large number of reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space which could include furniture and people, and air. 

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

The walls and ceiling of an auditorium are covered with a fibrous material to

  1. Make the auditorium fire proof

  2. Absorb sound and prevent echoes

  3. Give an ethnic look to the auditorium

  4. To prevent leakage of water from the roof
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

  We know that soft , porous and fibrous materials are good absorbers of sound , so they are used to the walls and ceiling of an auditorium to absorb the sound .

    If this is not done , sound waves after reflections (echoes) from walls and ceiling , will interfere and due to this at some places in auditorium , sound will not be heard (destructive interference) and at some places intensity of sound will be very large (constructive interference)

Multiple choice physics study of sound reverberation applications of reflection of sound multiple reflection of sound and reverberation applications of ultrasound

What will happen if the reverberation time in a big hall is too long?

  1. Sound will persist for some time in the hall

  2. The reflection may become an echo.

  3. Multiple reflection may occur

  4. All the above will happen
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The multiple reflection of sound from the walls of a room or hall causing sound to persist for some time is called reverberation. If the hall is too long , the reflection will become an echo.